Kate Becker: Shadows can be positively illuminating

CORRECTION: This column incorrectly described the process by which a solar eclipse occurs. They happen when the moon covers the disk of the sun, casting a shadow on the Earth.

What can you learn from a shadow?

An eclipse is nothing more than a shadow: We see a solar eclipse when the moon covers the disk of the Sun, casting a shadow on the Earth. When Earth's shadow falls across the face of the moon, we see a lunar eclipse.

Eclipses motivated ancient astronomers to understand the relationship of the sun, Earth and moon. In the 19th century, astronomers snapped their first photographs of the solar corona with a little help from the moon's shadow, which handily blocks out the disk of the sun while leaving the hot halo of the corona still visible. And in 1919, astronomers used a solar eclipse to test Einstein's prediction that gravity from a massive object like the sun can bend the path of starlight, providing a dramatic confirmation of general relativity.

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Today, astronomers use shadows to discover planets orbiting stars beyond our sun. As an exoplanet passes in front of its star, it casts a tiny shadow that makes the star appear minutely dimmer to anyone who happens to be looking at it from the proper angle. The Kepler space telescope, launched in 2009, exploits this fact by keeping constant watch on more than 100,000 stars and measuring their brightness so sensitively that it can pick up changes of less than one hundredth of a percent, making it precise enough to detect the shadow of an Earth-sized planet. The technique works so well that Kepler has already found more than 2,321 planet candidates.

Yet a shadow can reveal even more. By measuring exactly how long each mini-eclipse lasts and how often it occurs, astronomers can infer the presence of additional planets and maybe even moons, even if they don't cause eclipses of their own. That's because, even if we can't see these objects, they exert a gravitational tug on the orbit of the planet we can see. Just as 19th-century astronomers predicted the existence of planet Neptune, which they couldn't see, based on anomalies in the orbit of Uranus, which they could, astronomers can now deduce the existence of unseen worlds based on nothing more than the aberrations of shadows.

In early June, astronomers will get front-row seats for a mini-eclipse inside our own solar system as Venus casts its shadow across the sun. Transits of Venus are rare -- the next one won't be until 2117 -- and this one will give astronomers a chance to test whether they can take advantage of the eclipse to tease out details of Venus' murky atmosphere. In one experiment, astronomers will be applying a series of color filters to their telescopes to create a template that can one day be applied to other exoplanets to determine whether they, like Venus, have thick, hazy atmospheres.

Another team will watch the transit using the Hubble Space Telescope. Enlisting three different instruments to capture light across a wide range of wavelengths, they will try to read out the composition of Venus' atmosphere by comparing the spectrum of sunlight during the transit to the sun's normal spectrum. At first, this might seem redundant -- after all, we already know what Venus' atmosphere is made of, and this technique is already being applied to some exoplanets -- but the transit will give astronomers a chance to confirm that their technique is working as they expect.

Shadows are defined by darkness. Yet, when it comes to understanding the universe and our place in it, it turns out that shadows can be positively illuminating.

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